1. Field of the Invention
The present invention relates to the preparation of aqueous dispersions of copolymers by emulsion polymerization of ethylenically unsaturated monomers in the presence of an organosulfur chain transfer agent.
2. Description of the Prior Art
Aqueous dispersions of copolymers and terpolymers such as the styrene/butadiene, styrene/acrylates, styrene/acrylonitrile and acrylonitrile/butadiene/styrene latexes are widely used for various applications, including coating compositions and adhesive and impregnating compositions.
In order to limit the molecular weight and/or the degree of crosslinking of the resulting copolymer, which have profound effects on the ultimate uses of the latexes, it is usual to employ a chain transfer agent which functions during the polymerization process. The transfer agents typically employed industrially comprise aliphatic mercaptans and halogenated hydrocarbons. These two types of regulators present certain disadvantages, e.g., dispersions produced in the presence of a mercaptan, although having a very low residual mercaptan content, have an undesirable characteristic odor. If halogenated organic compounds, such as carbon tetrachloride, bromoform or bromotrichloromethane, are used, the residual contents thereof are relatively high. The odor associated with the mercaptans, or the presence of residual halogenated compounds, constitutes a serious limitation on the use of the dispersions for certain applications, for example as binders in adhesive or coating formulations, and, in particular, in formulations adopted for the coating of paper. These disadvantages may be eliminated either by treatment of the latex with an agent which converts the thiol to an odorless product (see, for example, U.S. Pat. No. 3,756,976) or by a devolatilization in the case of the halogenated organic compounds. However, these processes are expensive and difficult to carry out.
Other organic disulfides belonging to such families as diaryl disulfides, alkylthiuram disulfides and xanthogen disulfides are generally known as agents which regulate the molecular weight during the polymerization of unsaturated monomers including styrene and butadiene.
However, the particular compounds, diphenyl disulfide, tetramethylthiuram disulfide and diethylxanthogen disulfide, all present disadvantages. Diphenyl disulfide is insufficiently effective, the thiuram exhibits a retarding or even an inhibiting action on the polymerization and the xanthogens have a pronounced odor.
Diphenyl disulfides substituted by amino radicals have also been described as transfer agents for the polymerization and the copolymerization of styrene (FR-A-2,321,519); these species still present the disadvantage of being polymerization retardants.